The goal of this research is to gain a better understanding of the regulation of skin development. As self-renewing structures, the epidermis and hair follicles require a balance between keratinocyte cell division and terminal differentiation for their formation and maintenance. To gain insight into this process, the applicants are studying the function of Whn (Winged-helix nude), the product of the mouse nude locus and a putative member of the winged-helix/forkhead family of transcription factors. Loss-of-function mutations in whn result in the nude phenotype, which is characterized by the failure to produce visible hair, the abnormal formation of the epidermis, and the absence of a thymus. Based on their previous studies, the applicants have constructed a model to explain Whn's role in skin morphogenesis. They find that whn expression is induced, as keratinocytes arrest growth and initiate terminal differentiation. They postulate that Whn: 1) promotes the transition from proliferation to differentiation by activating genes associated with the differentiation program; and 2) induces differentiating cells to secrete growth factors. These growth factors then stimulate neighboring keratinocytes, which lack the Whn protein, to proliferate. As a result of this paracrine mechanism, a differentiating keratinocyte is replaced as it leaves the proliferative compartment, and the balance between multiplying and differentiating cells is maintained. The aim of this proposal is to test this model and provide a basic framework for the elucidation of Whn's mechanism of action. To assess Whn's direct and paracrine effects, transgenic mice will be generated in which Whn expression is specifically targeted to either proliferating or differentiating keratinocytes. Since the investigators have acquired preliminary evidence that Whn stimulates the expression of transforming growth factor-a (TGF-a), the ability of Whn to bind to the TGF-a promoter and activate transcription will be evaluated. Lastly, they and others have isolated the human homolog of whn, and find that the human and murine Whn proteins are 85 percent identical. As a first step in the characterization of this homolog, the Whn expression pattern will be examined in normal and diseased human skin. Given the pleiotropic effects of murine whn, the human homolog they believe could play a role in diseases associated with hyperproliferation, hair loss, or aberrant differentiation.